BEST FACE FORWARD: The exterior of the Barclays Center Paul Martinka/ photographer

Hoop fans take notice – an arena is truly growing in Brooklyn.

Workers earlier this month buttoned up the top of the under-construction Barclays Center in Prospect Heights by completing its steel-roof deck, and they also recently began insulating the rooftop and waterproofing it with a light-gray covering.

“This is that magic time when the building really begins to take shape, so that you can finally feel it’s an arena,” said Bob Sanna, executive vice president for construction at Forest City Ratner Cos., developer of the soon-to-be Brooklyn Nets’ 18,000-seat arena.

SHAPING UP: The dome has been completed at the Barclays Center at Flatbush and Atlantic Avenues. (Paul Martinka/ photographer)

The interior of the under-construction Barclays Center, new home of the Nets. (Paul Martinka/ photographer)

Brooklyn’s bustling downtown is seen through windows at the under-construction Barclays Center (Paul Martinka/ photographer)

Sanna during an interview with the Post yesterday said about 35 percent of the 188,425-square-foot roof is complete and that Barclays Center is still on schedule to open in September.

“We got lucky with the warm weather we’ve been having. You usually aren’t able to get roofers on a roof in February,” said Sanna.

Although it will be months before the Nets’ future hardwood-playing floor is installed, much of the arena’s interior is starting to shape up.

Premium seats are now being installed in the arena’s lower bowl. The upper-bowl’s seating is already complete, along with the lower and upper concourses.

The arena’s entrance to an abutting transit hub is also ready for game day tip-offs.

National Grid earlier this month began powering the site, so it could be heated, allowing temperature-sensitive work like floor- and wall-tiling to begin.

Next month, workers will begin work on the exterior’s most eye-grabbing feature — a canopy with an oval window called the “Oculus” in its center. It will extend out 80 feet from the arena entrance and hover 36 feet above a public plaza.

A video screen that can display everything from game highlights to art installations will wrap the Oculus’ perimeter.